1. Field of the Invention
The present invention relates to a pneumatic cylinder provided with a cushion mechanism by braking and stopping a piston at stroke end.
2. Prior Art
In the past, a pneumatic cylinder has been known, which is provided with a cushion mechanism for braking and stopping a piston at stroke end. Such pneumatic cylinder generally comprises a cylinder tube, a pair of covers for blocking both ends of the cylinder tube, a piston reciprocally moving within the cylinder tube, a piston rod integrated with the piston, a supply and discharge port for supplying and discharging compressed air to and from cylinder chambers on both sides of the piston, and a relief valve for limiting flow rate of exhaust air discharged from the cylinder chambers through the supply and discharge port, whereby the piston is decelerated by operating the relief valve and limiting flow rate of exhaust air when the piston approaches stroke end and braking and stopping the piston at stroke end.
However, the conventional type pneumatic cylinder as described above is disadvantageous in that the relief valve is directly incorporated in the cover, and this leads to more complicated structure of the cover and to inconveniences in manufacture.
Also, the relief valve is designed in such manner that it is opened by pressure of exhaust air discharged from the cylinder chambers through the supply and discharge port and that the pressure/relief pressure) when opening can be adjusted. Since the relief valve is provided at a predetermined position on the cover, it is often difficult or even impossible to adjust relief pressure depending upon site and position of the pneumatic cylinder, and this also results in inconveniences and troubles in maintenance or parts replacement.
Further, because the relief valve is opened by exhaust pressure, it is very likely to be influenced by exhaust air flow or by pressure fluctuation of the exhaust air. As a result, opening or closing conditions are less stabilized and flow rate of exhaust air is fluctuated. This further leads to bounding of the piston while moving and to impairment of stable cushion effect.